Oxidation of Benzoic Acid by Electrochemically Generated Ce(IV)

Abstract

This paper presents a study of the oxidation of benzoic acid in aqueous solutions by Ce(IV), which functions as an electrogenerated mediator. In this reaction, benzoic acid is oxidized to CO2, and the reaction displayed pseudo-first-order kinetics for the conditions 0.15-0.45 mM Ce(SO4)2, 0.5-3.0 mM benzoic acid, and 0.2 M H2SO4 at 25 degrees C. The k1 values were in the range 0.40 x 10(-4)-0.59 x 10(-4) s(-1) and increased with temperature within the range studied, 25-45 degrees C. The activation parameters for the reaction, deltaH++, deltaG++, and deltaS++, were determined to be 10.5 kcal/mol, 23.3 kcal/mol, and -43 cal/(mol K), respectively. The electrolysis of Ce(III) in a stirred solution at 25 degrees C and with 1.5 V constant potential generated a 96% Ce(IV)-4% Ce(III) distribution in 140 min. The k for the generation of Ce(IV) was 0.0003 s(-1). Electrolysis under similar conditions but with a large excess of benzoic acid in solution produced a steady-state conversion of 87% Ce (IV) and the corresponding k was 0.00027 s(-1). This reduction in k is consistent with the relative magnitudes of the oxidation reactions for Ce(III) and benzoic acid. Correspondingly, the electrolysis of benzoic acid (0.594 mM) in a 0.2 M H2SO4 solution containing 0.25 mM Ce2(SO4)3 was accomplished by applying 1.5 V to the stirred solution at 25 degrees C. The benzoic acid concentration was reduced to 0.026 mM, or by 96%, in 456 h. In this experiment the potential was applied for 3 h and then turned off for 48 h, and the sequence was repeated 8 times.